The present invention is directed to electronic amplifiers, particularly those of the type employed in automatic equipment for testing electronic circuitry.
In many electronic circuits, some of the circuit terminals can act alternately as input terminals and as output terminals, and they often change rapidly between these modes. In order to test such circuits, therefore, the test equipment must have corresponding common terminals that can be used both to drive the terminals of the circuit under test and to sense the signals on those terminals. The changing between sensing and driving functions must occur very rapidly in some cases, and this can present difficulties in the test circuitry.
Specifically, the driver amplifier, which applies the output signal from the test equipment to the device under test, is required to change rapidly from a high-impedance state to a driving condition, and this can cause transients that should not be applied to the device under test. In addition, while the driver amplifier is in its high-impedance state but still connected to the terminal of the device under test, that terminal could carry voltages that would cause the transistors in the amplifier to break down.
A design parameter of particular concern in sensor amplifiers generally, including those in driver-sensor combinations, is the input offset current. Offset current at the input terminal of a sensor amplifier can affect the sensed signal, so input offset current should be kept to a minimum.
In one method used in the past to minimize input bias current, the collector current of an amplifier's input transistor is supplied by the emitter of a collector-circuit transistor that is identical to the input-stage transistor and is connected between the supply rail and the collector of the input-stage transistor. Because the emitter current of this collector-circuit transistor is the same as the collector current of the amplifier transistor, the base currents of these two transistors are nearly equal. A current-mirror circuit produces a current equal to the base current of the collector-circuit transistor and feeds it to the base of the amplifier's input transistor. Since the input transistor receives all of its base current from the current mirror, very little current is drawn from or supplied to whatever circuit drives the amplifier input terminal.
While this arrangement can be quite effective in reducing input offset current, it is appropriate only for monolithic circuits having only one type (npn or pnp) of relatively fast transistor; it is not readily adapted to an amplifier whose input stage comprises complementary transistors.
It is accordingly an object of the present invention to change between the driving and high-impedance states of a driver amplifier at a rapid rate, with a minimum of transients, and without the danger that terminal voltages will cause driver-amplifier transistors to break down when the amplifier is in its high-impedance state.
It is another object of the invention to minimize input offset current in an amplifier having a complementary-transistor input stage.